1. Field of the Invention
The present invention relates to sensors for the detection of metallic debris in dielectric fluids using electromagnetic fields and more particularly to a sensor system having improved reliability which is adapted to function in difficult environments.
2. Description of the Prior Art
Sensors are known for detecting metallic debris in fluid flow passageways such as engine and transmission lubricating systems where the presence of metal particles in the dielectric fluid lines can be an indication of the impending failure of the system. Sensing systems of this type are disclosed in U.S. Pat. Nos. 4,926,120 and 5,041,856, issued to W. A. Veronesi et al. and assigned to United Technologies Corp., and in co-pending application U.S. Ser. No. 07/974,242 of Lev Sorkin, now U.S. Pat. No. 5,357,197 assigned to the same assignee as the present application. The patented systems use resonant (inductance-capacitance) tank circuits to sense the presence of metallic debris in oil or similar fluids. The fluid under test passes through the center of a probe which produces a field configuration with a magnetic component that is easily disturbed by metallic particles passing through its center. Changes to the probe's impedance characteristics are caused by the presence of metallic particles, and are registered by a BRIDGE CIRCUIT and in turn are read out and counted to determine the extent of foreign metallic particles in the oil. This determination may be a prediction of impending failure of machinery such as engines and transmissions.
As shown particularly in FIG. 3 of U.S. Pat. No. 4,926,120, the probe may consist of a TRANSFORMER 19, 20, 21, a TANK CIRCUIT 12, 16, 18, and a VARIABLE RESISTOR 34. The remainder of the components generally comprise the BRIDGE CIRCUIT (including readout and tabulating circuitry).
The TANK CIRCUIT consists of one turn (two turns in U.S. Pat. No. 5,041,856) of electrically conductive material which forms an inductor coil 12. The coil 12 has a clear center big enough for the flow of the dielectric fluid under test for debris. The inductor 12 is brought into resonance with one or several capacitors 16 which combine with it to form an L/C tank circuit. The BRIDGE CIRCUIT has two control loops that:
1) seek to excite the tank circuit at its resonant frequency by automatically tuning a VOLTAGE CONTROLLED OSCILLATOR 22 to this resonant frequency; and
2) adjust the variable resistor (34) so that its resistance equals the EQUIVALENT PARALLEL RESISTANCE of the tank circuit.
When this is done the output of the BRIDGE CIRCUIT is zero and both loops settle into a stable equilibrium. This stable point is perturbed by the passage of ferrous and non-ferrous foreign debris differently, allowing a tally of such particle presence to be made.
The probe operates at an extremely low electrical impedance level, typically less than 1 ohm for each capacitive and inductive reactance. At resonance these cancel (to the extent that losses allow), producing a resistive impedance on the order of 100 ohms. The BRIDGE CIRCUIT is set up to balance its output to zero at this impedance. The INTERCONNECT 17 must be kept extremely short, on the order of 6 inches, so as not to cause a total loss of signal when driving the low, off-resonance impedance of the tank. This requires that the TRANSFORMER, TANK, and VARIABLE RESISTOR all be located in close proximity, which typically causes them to be exposed to engine heat or other harsh environment. In particular, in the preferred embodiment, the VARIABLE RESISTOR 34 is a silicon field effect transistor that cannot long survive exposed to the typical 300.degree. C. engine heat encountered in this system. Consequently, high temperature FETs and ferrites for the TRANSFORMER are required.
A further problem is posed by the fact that existing probes or monitors must be installed in a conducting housing, typically of aluminum. Such a housing acts as a short-circuited transformer secondary winding which reduces the sensor Q value and increases loss of sensitivity. Thus, sensitivity to small non-ferrous particles is compromised.
Accordingly, it is an object of the present invention to provide an improved metallic sensor with a probe that overcomes the environmental and reliability problems of the prior art sensor systems.
It is another object of the invention to provide a probe that operates at a higher electrical impedance level to render it capable of remote location from the other components of the sensor system.
It is a further object of the invention to provide a probe and sensor system that may be constructed of less expensive materials while capable of performance comparable to sensors of the prior art.